The oscillator is designed to be resonant at approximately 25 megahertz. C1, the trimmer capacitor, is used to adjust the oscillator to a point close to where it will drop out of this basic resonant frequency. This is, of course, adjusting the sensitivity. When the oscillator is in resonance the operating level at TP2 and the filtered version at TP3 are at approximately -8 volts. When a hand-held wire touches the detector button the added capacitance drops the oscillator out of resonance to a much lower sawtooth shaped oscillation. The element detected, through, is the change in operating level (to -6 v approximately 2 volts) of the circuit at TP2 and TP3 (Incidentally, for a very short wire the primary source of the capacitance is the person holding the wire. If they squeeze the wire harder it increases the capacitance and, of course, the sensitivity or ability to strip an even smaller wire.) The change in level at TP3 is amplified approximately 16 times by the inverse amplifier stange. Since adjusting C1, the real threshold or sensitivity adjustment, will cause the operating level to change at TP2/TP3, the inverse amplifier's output level, in the quiescent state, must be kept near the +10 volt level. That is the purpose of totentiometer R1. Simply, adjusting C1 interacts with TP4 the output of the inverse amplifier. The level of +20 volt was chosen to allow as much negative swing, which triggers the one-shot, as possible. The timing circuit was a requirement because without it the circuit is fast enough that when the stripper operates it mechanically grips the wire and pulls the blades backward removing the stripped insulation and simultaneously removing the detector button and sensor connection. This allows the oscillator to quickly return to resonance and in effect arms the circuit for a retrigger. It will retrigger producing a machine gun effect. The delay allows the operator to withdraw the wire without a retrigger. The delay disarms the trigger output.
The prior art patent literature has included noncapacitance type wire contact switch means for actuating terminal crimping devices as shown in U.S. Pat. Nos. 2,775,283; 2,889,422; and, 3,004,581. The patent literature also includes capacitance type contact switching circuits as shown in U.S. Pat. Nos. 3,571,666; 3,725,808; 3,927,336; and, 4,169,982. A capacitance type contact sensing circuit utilized for sensing interruptions in a moving fiber strand has been disclosed heretofore as shown in U.S. Pat. No. 3,438,189.
In contrast, it is an object of the present invention to utilize capacitance tough wire sensing for starting a timer circuit to activate the wire stripper by engaging a solenoid valve.
In accordance with a preferred embodiment of the present invention, a touch sensor for wire stripper devices comprises a picofarad detector circuit responsive to initial contact wire change capacitance and incorporates an oscillator circuit, an inverse amplifier circuit, and timer circuit for activating a relay driver circuit.